1. Field of the Invention
This invention relates to optical information processing and, more particularly, to the sorting and classification of radio frequency signals and the excision of interference.
2. Discussion of the Prior Art
A given signal waveform usually consists of the signal of interest and interference. The signal and interference composite is a current or voltage waveform which is a function of time. Usually, only the signal without interference is of concern, and more specifically, only certain frequencies of the given signal are of concern; therefore, it is necessary to extract the useful information and isolate the desired frequencies from the composite signal.
A receiver has the job of extracting source information from a received modulated signal which has in some form been corrupted by interference. A modulated signal is originally a bandpass signal with a given carrier frequency that has been encoded with source information by the process of modulation. Standard receivers employ the superheterodyne receiving technique. This technique consists of either up-converting or down-converting the input signal to some convenient frequency band, and then extracting the encoded information by using the appropriate detector. Filtering in the superheterodyne receiver is done at the original radio frequency and at the up or down converted frequency.
It has been shown that maximum probability of detection of a signal occurs when it is processed with a matched filter. Basically, the purpose of a matched filter is to characterize an incoming waveform based on a predetermined signal waveshape as determined by the transfer function of the matched filter. In other words, a matched filter is used to detect the presence of a particular signal based on the characteristics of that particular signal. The objective of a matched filter system is to weight the input signal waveform and filter the interference so that at the sampling time the output signal level will be as large as possible with respect to the Root Mean Square (RMS) of the output noise level. A matched filter is therefore a linear filter that minimizes the noise level while maximizing the signal level. In applications employing matched filter filtering systems, the signal may or may not be present, but if it is present, its waveshape must be completely or nearly completely characterized. When the waveshape is not known, or not completely characterized, a search for a signal generally means that conditions for intercept will be less than ideal. That is, the filter pass band must be large enough to pass all frequencies of all possible signals with the most generalized response, clearly not a matched filter.
Broadband receivers often have to function in dense signal and noise environments. The job of a filtering system or receiver is to sort, classify and remove interference, such as noise, from signals of interest (SOI). The interference may be characterized as noise from the outside world and disturbances from the electronic components as well as mechanical components of the system itself. To enhance the filtering function it would be highly desirable to perform real-time parallel signal processing to effectively channelize and remove unwanted spectra from the SOI's. This would in effect transform a single channel that is corrupted with unwanted signals, interference and noise to a specific number of discrete channels where each is tailored to selected SOI's.
The accepted practice has been to electronically filter the given signals. It has been found, however, that electronic filtering means have a number of distinct disadvantages. Inherent in electronic filters is the problem of phase distortion and non-linear effects. In addition, electronic filters are also susceptable to noise corruption; therefore, other techniques such as optical filtering of signals have increasingly become of interest.
The prior art shows a number of examples of optical systems for the detection and/or removal of interference and unwanted frequencies from given SOI's. U.S. Pat. No. 4,645,300, assigned to the same assignee as the present invention, discloses a Fourier Plane Recursive Optical Filter which is used to recursively pass a given signal beam a multiplicity of times through a single optical spatial filter such that the attenuation of unwanted signal frequencies is multiplied. However, the aforementioned invention does not exploit the advantages of parallel processing.
U.S. Pat. No. 3,671,106, discloses an Optical Multiplex Filter System which is used for optical data processing. The object of the invention is to distribute data over a large number of parallel transmission channels in which the data passes through optical filters that process the data in a described manner. However, the aforementioned invention does not provide the filtered signals at their original radio frequencies.
U.S. Pat. No. 4,699,466, assigned to the same assignee as the present invention, discloses an Optical RF Filtering System which is used as an optical notching filter in which a band of radio frequencies such as the RF input of a radio receiver is an input signal. This invention provides adaptive programmable spatial filtering techniques in which an optical filter is programmed to notch out spikes. However, the aforementioned patent does not provide matched filter capabilities.